The present invention relates to integrated electro-optic components and, in particular, to an electro-optic component having a channeled waveguide pattern therein, the component being fabricated on a substrate of potassium titanyl phosphate.
There has been much current interest in developing optical communication systems. By using optical waves to carry messages instead of electrical signals the bandwidth of communication is increased. The basic element of these systems is the optical waveguide which transmits or conducts waves of optical frequencies from one point to another and can serve to connect various devices based on optical integrated circuit technology. The optical waveguide is an optically transparent medium surrounded by media with lower indices of refraction so that light propagating along the waveguide is highly reflected at the boundaries of the surrounding media, thus producing a guiding effect. The most frequently used material for such a waveguide is high purity silica in the form of a fiber.
For use in telecommunications it has been necessary to develop optical waveguide structures which can couple, divide, switch and modulate the optical wave as it propagates through a network. Optical waveguide structures have been designed and fabricated in suitable substrates for the manipulation of guided light. Electro-optic responsive substrates such as zinc selenide, lithium niobate and more recently potassium titanyl phosphate have been used.
U.S. Pat. No. 4,070,094 (Martin) discloses an optical waveguide interferometric modulator (or "Mach-Zehnder") configuration formed on ZnSe by cadmium diffusion into the etched waveguide pattern of a silicon dioxide mask to form controllable refractive index increases in the ZnSe substrate. U.S. Pat. No. 4,447,116 (King et al.) and U.S. Pat. No. 4,266,850 (Burns) both disclose optical waveguide structures of the interferometric modulator (or "Mach-Zehnder") configuration of titanium diffused lithium niobate.
U.S. Pat. No. 4,012,113 (Kogelnik), U.S. Pat. No. 4,211,467 (Cross) and U.S. Pat. No. 4,645,293 (Yoshida et al.) disclose optical waveguide structures of the optical coupler configuration formed on titanium diffused lithium niobate.
U.S. Pat. No. 4,639,074 (Murphy) discloses a fiber optic coupling system for a lithium niobate chip. In this system the fiber is allowed to butt directly against an end face of the waveguide substrate.
One can calculate requirements of a waveguide to transmit light through a change of direction by utilizing the papers of E.A.G. Marcatili (references: E.A.J. Marcatili, The Bell System Technical Journal, Volume 48, Number 7, pages 2071-2102 and pages 2103-2132, September 1969.)
Although presently the predominant material of choice for these structures is lithium niobate, lithium niobate suffers several disadvantages. The optical wave propagation properties poorly match its microwave propagation characteristics. As a result, in devices such as interferometric modulator with traveling wave electrode structures, optical bandwidth is limited by a large difference in the speed of the optical wave and the externally applied microwave. Using a lithium niobate substrate the effective refractive index at optical frequencies is approximately 2.2 whereas it is approximately 4.2 at microwave frequencies. Haga et al., IEEE J. Quantum Elect QE-22, Number 6, 902-906, 1986. There exists a need for a modulator which has both a broad optical bandwidth and more efficient use of electrical power.
Further, lithium niobate is increasingly photorefractive at optical wavelengths significantly below 1.3 micrometers. This property leads to unacceptable device instability at optical power levels required for many applications. The high dielectric constant of lithium niobate makes it difficult to build high impedance devices to efficiently utilize microwave power.
These disadvantages of ligthium niobate have been avoided by the use of potassium titanyl phosphate (KTP) as the substrate material. Copending applications Ser. No. 001,417, filed Jan. 8, 1987, and a continuation-in-part thereof, Ser. No. 129,058 filed Dec. 11, 1987 disclose a process for producing an optical waveguide and the product produced thereby wherein the crystal substrate is KTP.
Advantages of the potassium titanyl phosphate substrate include a better match between the optical and microwave effective indices (1.8 and 2.5 respectively) leading to a lessened velocity mismatch between microwave and optical signals. It is believed advantageous to utilize this feature of KTP in a guided wave device with a traveling wave electrode structure having greater optical bandwidth and requiring less microwave power to drive it. KTP exhibits good photorefractive stability not only for near infrared optical waves where lithium niobate is employed but also at all wavelengths down to the violet. Due to the lower microwave dielectric constant of KTP, design of higher impedance traveling wave electrodes is practical.